Deferrability Analysis for JavaScript

Modern web browsers allow asynchronous loading of JavaScript scripts in order to speed up parsing a web page. Instead of blocking until a script has been downloaded and evaluated, the async and defer tags in a script allow the browser to download the script in a background task, and either evaluate it as soon as it is available (for async) or evaluate it in load-order at the end of parsing (for defer). While asynchronous loading can significantly speed up the time-to-render, i.e., the time that passes until the first page elements are displayed on-screen, the specification for correct loading is complex and the programmer is responsible for understanding the circumstances under which a script can be loaded asynchronously in either mode without breaking page functionality. As a result, many complex web applications do not take advantage of asynchronous loading. We present an automatic analysis of web pages which identifies which scripts may be safely deferred, that is, deferred without any observable behavior on the page. Our analysis defers a script if every other script that has a transitive read or modification dependency does not access the DOM. We approximate access and modification sets using a dynamic analysis. On a corpus of 462 professionally developed web pages from Fortune 500 companies, we show that on average, we can identify two or three scripts to defer (mean; median: 1). On 18 pages, we find at least 11 deferrable scripts. Deferring these scripts can have notable impact on time-to-render: in 49 pages, we could show that the median improvement in time-to-render was at least 100ms, with improvements up to 890ms.